Arc plasma device having a thimble-shaped electrode of pyrolytic graphite



June 6, 1967 H HAHN 5,324,333

ARC PLASMA DEVICE HAVING A THIMBLE-SHAPED ELECTRODE OF PYROLYTIC GRAPHITE Filed June 18, 1965 INVENTOR. HENRY HAHN AGENT United States Patent ARC PLASMA DEVICE HAVING A THIM- BLE-SHAPED ELECTRODE 0F PYROLYTIC GRAPHITE Henry Hahn, Fairfax, Va., assignor to Curtiss-Wright Corporation, a corporation of Delaware Filed June 18, 1965, Ser. No. 465,086 4 Claims. (Cl. 313-231) This invention relates to a means of producing a high temperature electrical plasma, and more particularly to a plasma device having a novel electrode system and cooling means, whereby the efficiency of the device and its operating life are increased over what has previously been known. I

Devices are known in the prior art for producing a jet of high temperature electrical plasma for use as a torch or for other purposes. An example is US. Patent No. 2,587,331 to Jordan, wherein there is disclosed apparatus having a longitudinal metallic electrode surrounded by a coaxial cylinder having an annular metallic electrode adjacent to the tip of the center electrode. A flow of gas is provided through the cylinder, which gas discharges through the annulus and is ionized by electrical discharge between the tip of the center electrode and the annular electrode, the ionized plasma passing through the aperture in a flame-like jet.

Because of the intense heat generated by the electrical discharge, such metallic annular electrodes require cooling, usually by a flow of water through internal passages, the water thenbeingsent either to a heat exchanger or to waste. In either case, water-cooling of the annulus takes a great deal of heat out of the system, which results in high power consumption to maintain the temperature of ionization. Even with the best water-cooling the life of the annular electrode is limited to a rather short operating time, owing-to erosion of the metal by the plasma jet passing through the aperture, since the coefficient of heattransfer in metals is not high enough to maintain the surface exposed to the plasma cool enough to with stand such erosion. j

,The present invention overcomes these disadvantages of the prior artby providing an apparatus of the character described having an annular front electrode of pyrolytic graphite, so formed and having its conductive direction so oriented as to conduct heat away from the region of highest temperature more rapidly than any metal is capable of, the heat being then conveyed to a location where it is used to preheat the incoming gas. Thus the heat can be conserved within the system, reducing power consumption and protecting the orifice of the annular electrode from erosion.

It is therefore a primary object of this invention to provide an improved electrical plasma device.

Another object is the provision of a novel electrode system for a plasma device.

It is a further object of the invention to provide an electrical plasma system with reduced heat losses and reduced power consumption.

The foregoing objects and others ancillary thereto will be readily understood on reading the following specification in connection with the accompanying drawings, in which FIG; 1 is an elevational cross-section along the longitudinal axis of the novel plasma device; and

FIG. 2 is a similar cross-section of a modified embodiment of the annular electrode.

In FIG. 1 there is shown generally a suitable means of producing a high frequency gaseous plasma discharge, such as the coaxial transmission line 11, comprising the outer and inner hollow cylindrical conductors 12 and 13 respectively, energized by high frequency oscillations Patented June 6, 1967 from any suitable power source of known type, shown schematically at 14 and electrically connected to the transmission line by leads 16 and 17. A flow of gas is provided through cylinder 12 for directing the thermally active portion of a high frequency electrical discharge from the tip of conductor 13 into a flame-like jet. There is shown a gas source 18 providing the selected gas at a suitable pressure; about 20 pounds per square inch is a generally usable pressure, but the pressure used may vary within wide limits according to the use to which it is desired to put the device. Argon, helium, and

other monatomic gases have been found satisfactory, as'

well as nitrogen and other molecular gases which are dissociated by electrical energy during ionization and.

The inner hollow cylindrical conductor 13 is positioned coaxially within the outer cylinder 12 by any suitable means, such as the insulating positioner 21, which .may be formed of glass or other appropriate insulating material. It is convenient to form positioner 21 as a solid annular piece closely fitting cylinders 12 and 13, in order to block flow of gas in the rearward direction and require it to discharge through the nozzle. If the positioner 21 is a spider, or is porous or otherwise apertured, other closure means must be provided. The front or discharge end of conductor 13 terminates in a generally frustoconical electrode holder 22 which cooperates with an annular electrode later to be described. Holder 22 secures and positions an electrode 23 which projects forwardly thereof.

Electrode 23 may be formed of metal or carbon, but is preferably formed of pyrolytic graphite having its heatconducting direction longitudinally oriented along the axis, in order to conduct heat away from the region of the ionizing discharge at its tip.

The inner conductor 13 is cooled by any suitable means,

as is known in the prior art. The arrangement shown is a conventional squirt-tube system, in which conductor 13 is provided with an internal delivery tube 24, positioned by an appropriate fitting 26 mounted within conductor 13. A water intake tube 27 transpierces the walls of the outer and inner conductors and communicates through fitting 26 with delivery tube 24, which has an open end Within the hollow interior of electrode holder 22. The water thus delivered to the electrode holder returns through the annular space 28 surrounding the delivery tube within conductor 13, and discharges through outlet tube 29 communicating with the annular space 28 through the appropriately channeled fitting 26. Inlet tube 27 and outlet tube 29 are insulated from conductor 12 by insulating spacers 25 or other convenient means.

The front end of outer cylinder 12 bears an external mounting flange 30, to which is clamped an annular electrode-holder plate 31 by a second annular plate 32, which is attached to flange 33 by any conventional mounting means, such as screws 33. Holder plate 31 has an axial bore therethrough, in which is mounted by a slight press fit the annular electrode 34. Plate 31 is formed of an electrically conducting material which may be metal or carbon, but is preferably pyrolytic graphite having its heat-conducting direction radially oriented, in order to dissipate any heat which may reach it. However, a relatively minor amount of heat will reach plate 31, owing to the conductive properties of annular electrode 34.

Electrode 34 has a generally thimble-shaped configuration, having a rear wall portion of which the exterior surface is cylindrical and parallel to the longitudinal axis of the device, to provide a precise fit in the bore of the holder plate 31. The forward wall portion of electrode 34 is a curved nose tangent to the rear portion and having a cylindrical nozzle bore 36 therethrough coaxial with the cylindrical portion of the electrode. The cylindrical Wall portion of the electrode is tapered in thickness from nearly a knife-edge at the rear end to approximate tangency with the inner curvature of the nose portion. Such a tapered configuration provides a generally frustoconical chamber 37 within the annular electrode 34, which chamber receives the frustoconical electrode holder 22. Electrode 34 and holder 22 are so positioned as to be coaxial, with the frustoconical inner wall surface of electrode 34 being generally parallel to the frustoconical surface of holder 22.

The annular electrode 34 is formed of pyrolytic graphite, with its direction of heat conduction oriented as shown by arrows 38, radially outwardly from the nozzle bore 36, around the curvature of the nose, and longitudinally along the cylindrical portion of the electrode toward the rear end. Thus, heat conduction throughout is always parallel to the exterior surface of the electrode.

As is well known, pyrolytic graphite exhibits strongly anisotropic properties in heat conduction, such that in a flat plate it will conduct heat parallel to the plane of the plate, and insulate in the direction transverse thereto, that is, across the thickness. The present invention utilizes this anisotropic property by forming the annular electrode 34 of pyrolytic graphite with the direction of heat conduction curved congruently with the exterior surface of the electrode so that conduction is parallel thereto, and the electrode is therefore a heat insulator across its wall thickness.

Thus, it will be seen that in operation of the device the heat generated by ionization of the gas by high frequency discharge between the tip of center electrode 23 and the annular electrode 34 will be rapidly carried away from the nozzle 36 through which the ionized gas exhausts in a flame-like jet, around the curvature of the electrode, and rearwardly parallel to the outer surface. Incoming gas is forced to pass through chamber 37 between the two electrodes, and as it passes across the tapered inner surface of the annular electrode which is inclined to the direction of heat conduction, and is therefore the only surface of the electrode at which heat transfer can take place, the gas is preheated by heat carried away from the nozzle to that location. Because of such preheating, less energy is required to raise the gas to ionization temperature in the nozzle region. In mounting and positioning the two electrodes, care must be taken that the distance between the center electrode 23 and the annular electrode 34 shall be less in the nozzle region than the distance between holder 22 and the annular electrode in chamber 37, in order that the high frequency ionizing discharge shall take place at the nozzle.

The tensile strength of pyrolytic graphite greatly increases at elevated temperatures, and the material is inherently capable of withstanding higher temperatures than metals without damage. With the present apparatus having an annular electrode conductively oriented as described, it has been possible to greatly increase the continuous operating time over that feasible with similar devices having electrodes of either metal or ordinary graphite, and the over-all life of the device has been much extended.

Even with an annular electrode of pyrolytic graphite of the present type, there is eventually some erosion of the nozzle bore 36. A means of correcting for this condition is shown in FIG. 2. There is shown an annular electrode 34a of pyrolytic graphite, formed and oriented as previously described, but having a nose bore 39 of larger diameter than is desired for a nozzle. Inserted by press-fitting in the bore 39 is a cylindrical annular bushing 41, having a nozzle bore 36a therethrough. Bushing 41 is also formed of pyrolytic graphite, having its conductive direction oriented radially so as to convey heat away from nozzle 36a to the main portion of the electrode, wherein it will be carried around the curvature and rearwardly as before.

When the nozzle 36a of bushing 41 becomes sufficiently eroded the bushing may be pushed out of its bore and a new one installed. Alternately, one may begin operations with an electrode 34 having no bushing, and after a certain amount of erosion it may be bored and a bushing installed. However, it is convenient to have the bore 39 provided initially at the time of fabrication.

Although the invention has been described above in a preferred embodiment, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the inventive concept. It is intended to cover all such modifications by the appended claims.

What is claimed is:

1. An arc plasma device, comprising in combination a coaxial transmission line having inner and outer cylindrical members with nozzle means atone end thereof, means providing a gaseous flow between said members, means for imposing a high frequency potential between said members to ionize said gaseous flow in the nozzle region and to produce a flame-like discharge from the nozzle, wherein the improvement comprises: said outer member having at one end thereof a first electrode formed of pyrolytic graphite of generally thimble-shape to define a chamber therein communicating with the interior of said outer cylindrical member, said first electrode comprising at its inner end a first wall portion having a cylindrical outer surface and a generally frustoconical inner heat-transfer surface angularly disposed to the outer surface and at its outer end a second wall portion continuous with said first portion and curved to substantially close said outer end, said curved wall portion having a nozzle aperture therethrough coaxial with said outer cylindrical member, said inner cylindrical member having at one end thereof an electrode holder received Within said chamber of said first electrode, said holder having a generally frustoconical outer surface substantially parallel with the frustoconical inner heat-transfer surface of said first electrode and de fining an annular gas passage therewith, said holder bear-v ing a second electrode adjacent to said nozzle aperture and coaxial therewith, said pyrolytic graphite of said first electrode having its heat-conducting direction oriented,

parallel to the exterior surface of said thimble-shaped electrode to conduct heat away from said nozzle aperture and to said frustoconical heat-transfer surface, said gaseous flow passing through said annular gas passage and being preheated by removing heat from said transfer surface.

2. The combination recited in claim 1, wherein said outer cylindrical member has a closure member having an axial bore therethrough, said first electrode being mounted in said bore, said closure member being formed of electrolytic graphite having its heat-conducting direction radially oriented, said holder for said second electrode is water-cooled, and said second electrode is formed of pyrolytic graphite having its heat-conducting direction longitudinally oriented to transfer heat to said electrode holder.

3. An electrode for an arc plasma device, said electrode being formed of pyrolytic graphite and being generally thimble-shaped to define a chamber therein and comprising a first generally cylindrical wall portion having a longitudinal axis and open at one end and having a cylindrical outer surface parallel to said axis and a generally frustoconical inner heat-transfer surface angularly disposed to said cylindrical outer surface, and a second wall portion continuous with said first portion and curved tangent thereto and substantially closing the opposite end of said electrode, said second wall portion having a cylindrical nozzle bore therethrough coaxial with said first wall portion, said pyrolytic graphitehaving its heat-conducting direction oriented parallel to the exterior surface of said electrode to conduct heat away from said nozzle bore to said frustoconical heat-transfer surface.

4. An electrode for an arc plasma device, said electrode being formed of pyrolytic graphite and being generally thimble-shaped to define a chamber therein and comprising a first generally cylindrical wall portion having a longitudinal axis and open at one end and having a cylindrical outer surface parallel to said axis and a generally frustoconical inner heat-transfer surface angularly disposed to said cylindrical outer surface, and a second wall portion continuous with said first portion and curved tangent thereto and substantially closing the opposite end of said electrode, said second Wall portion having a first cylindrical bore therethrough coaxial with said first wall portion, a replaceable cylindrical bushing formed of pyrolytic graphite disposed in said bore and having a nozzle bore therethrough coaxial with said first wall portion, the pyrolytic graphite of said bushing having its heat-conducting direction radially oriented to conduct heat away from said nozzle bore to said first bore, the pyrolytic graphite of said thimble-shaped electrode having its heatconducting direction oriented parallel to the exterior surface of said electrode to conduct heat away from said first bore to said frustoconical heat-transfer surface.

References Cited UNITED STATES PATENTS 2,587,331 2/1952 Jordan 2l9-12l 2,944,140 7/1960 Giannini 219-121 2,967,926 1/1961 Edstrom 219-121 3,107,180 10/1963 Diefendorf 117226 FOREIGN PATENTS 1,233,796 5/1960 France.

JAMES W. LAWRENCE, Primary Examiner.

S. D. SCHLOSSER, Assistant Examiner. 

1. AN ARC PLASMA DEVICE, COMPRISING IN COMBINATION A COAXIAL TRANSMISSION LINE HAVING INNER AND OUTER CYLINDRICAL MEMBERS WITH NOZZLE MEANS AT ONE END THEREOF, MEANS PROVIDING A GASEOUS FLOW BETWEEN SAID MEMBERS, MEANS FOR IMPOSING A HIGH FREQUENCY POTENTIAL BETWEEN SAID MEMBERS TO IONIZE SAID GASEOUS FLOW IN THE NOZZLE REGION AND TO PRODUCE A FLAME-LIKE DISCHARGE FROM THE NOZZLE, WHEREIN THE IMPROVEMENT COMPRISES: SAID OUTER MEMBER HAVING AT ONE END THEREOF A FIRST ELECTRODE FORMED OF PYROLYTIC GRAPHITE OF GENERALLY THIMBLE-SHAPE TO DEFINE A CHAMBER THEREIN COMMUNICATING WITH THE INTERIOR OF SAID OUTER CYLINDRICAL MEMBER, SAID FIRST ELECTRODE COMPRISING AT ITS INNER END A FIRST WALL PORTION HAVING A CYLINDRICAL OUTER SURFACE AND A GENERALLY FRUSTOCONICAL INNER HEAT-TRANSFER SURFACE ANGULARLY DISPOSED TO THE OUTER SURFACE AND AT ITS OUTER END A SECOND WALL PORTION CONTINUOUS WITH SAID PORTION AND CURVED TO SUBSTANTIALLY CLOSE SAID OUTER END, SAID CURVED WALL PORTION HAVING A NOZZLE APERTURE THERETHROUGH COAXIAL WITH SAID OUTER CYLINDRICAL MEMBER, SAID INNER CYLINDRICAL MEMBER HAVING AT ONE END THEREOF AN ELECTRODE HOLDER RECEIVED WITHIN SAID CHAMBER OF SAID FIRST ELECTRODE, SAID HOLDER HAVING A GENERALLY FRUSTOCONICAL OUTER SURFACE SUBSTANTIALLY PARALLEL WITH THE FRUSTOCONICAL INNER HEAT-TRANSFER SURFACE OF SAID FIRST ELECTRODE AND DEFINING AN ANNULAR GAS PASSAGE THEREWITH, SAID HOLDER BEARING A SECOND ELECTRODE ADJACENT TO SAID NOZZLE APERTURE AND COAXIAL THEREWITH, SAID PYROLYTIC GRAPHITE OF SAID FIRST ELECTRODE HAVING ITS HEAT-CONDUCTING DIRECTION ORIENTED PARALLEL TO THE EXTERIOR SURFACE OF SAID THIMBLE-SHAPED ELECTRODE TO CONDUCT HEAT AWAY FROM SAID NOZZLE APERTURE AND TO SAID FRUSTOCONICAL HEAT-TRANSFER SURFACE, SAID GASEOUS FLOW PASSING THROUGH SAID ANNULAR GAS PASSAGE AND BEING PREHEATED BY REMOVING HEAT SAID TRANSFER SURFACE. 